It is now common that a cryptography technology is used for recording digital contents which consist of video and audio data on a recording medium, and selling the contents or distributing the contents via a network, to prevent a malicious use of the contents, such as tampering and tapping.
For example, according to CPPM (Content Protection for Prerecorded Media) standard, a provider of a content encrypts the content to generate an encrypted content, and distributes a recording medium on which the encrypted content is recorded. The playback apparatus decrypts the encrypted content using the device key stored therein to play back the content. The device key is unique to the playback apparatus.
Meanwhile, there is a demand for changing a cryptosystem to another when the cryptosystem used for encryption is broken or a new cryptosystem is developed. As such a technique, Patent Document 1 discloses a technique for changing cryptosystems in accordance with types of application and data.
For changing cryptosystems, it is necessary to change keys used for performing encryption. As a conventional technique relating to changing of keys, Patent Document 3 discloses a system in which each terminal apparatus prestores a plurality of keys and all the apparatuses changes keys to be used in accordance with the same rule.    Patent Document 1: Japanese Laid-open Patent Application Publication No. H10-320191    Patent Document 2: Japanese Laid-open Patent Application Publication No. 2002-50956    Patent Document 3: Japanese Laid-open Patent Application Publication No. 2002-290396